This allows context-independent code generated by TurboFan to be cached
in the optimized code map and reused across native contexts. Note that
currently this cache is still flushed at GC time.

R=bmeurer@chromium.org,mvstanton@chromium.org
TEST=cctest/test-compiler/OptimizedCodeSharing

Review URL: https://codereview.chromium.org/1208013002

Cr-Commit-Position: refs/heads/master@{#29313}